12 files changed, 179 insertions, 20 deletions
diff --git a/lib/rubygems.rb b/lib/rubygems.rb
index ac225ca70a..ba7cf273b9 100644
--- a/lib/rubygems.rb
+++ b/lib/rubygems.rb
@@ -18,6 +18,7 @@ require_relative "rubygems/compatibility"
 require_relative "rubygems/defaults"
 require_relative "rubygems/deprecate"
 require_relative "rubygems/errors"
+require_relative "rubygems/target_rbconfig"
 
 ##
 # RubyGems is the Ruby standard for publishing and managing third party
@@ -179,6 +180,8 @@ module Gem
 
   @discover_gems_on_require = true
 
+  @target_rbconfig = nil
+
   ##
   # Try to activate a gem containing +path+. Returns true if
   # activation succeeded or wasn't needed because it was already
@@ -397,6 +400,23 @@ An Array (#{env.inspect}) was passed in from #{caller[3]}
   end
 
   ##
+  # The RbConfig object for the deployment target platform.
+  #
+  # This is usually the same as the running platform, but may be
+  # different if you are cross-compiling.
+
+  def self.target_rbconfig
+    @target_rbconfig || Gem::TargetRbConfig.for_running_ruby
+  end
+
+  def self.set_target_rbconfig(rbconfig_path)
+    @target_rbconfig = Gem::TargetRbConfig.from_path(rbconfig_path)
+    Gem::Platform.local(refresh: true)
+    Gem.platforms << Gem::Platform.local unless Gem.platforms.include? Gem::Platform.local
+    @target_rbconfig
+  end
+
+  ##
   # Quietly ensure the Gem directory +dir+ contains all the proper
   # subdirectories.  If we can't create a directory due to a permission
   # problem, then we will silently continue.
@@ -450,7 +470,7 @@ An Array (#{env.inspect}) was passed in from #{caller[3]}
   # distinction as extensions cannot be shared between the two.
 
   def self.extension_api_version # :nodoc:
-    if RbConfig::CONFIG["ENABLE_SHARED"] == "no"
+    if target_rbconfig["ENABLE_SHARED"] == "no"
       "#{ruby_api_version}-static"
     else
       ruby_api_version
@@ -810,7 +830,7 @@ An Array (#{env.inspect}) was passed in from #{caller[3]}
   # Returns a String containing the API compatibility version of Ruby
 
   def self.ruby_api_version
-    @ruby_api_version ||= RbConfig::CONFIG["ruby_version"].dup
+    @ruby_api_version ||= target_rbconfig["ruby_version"].dup
   end
 
   def self.env_requirement(gem_name)
diff --git a/lib/rubygems/ext/builder.rb b/lib/rubygems/ext/builder.rb
index be1ba3031c..12eb62ef16 100644
--- a/lib/rubygems/ext/builder.rb
+++ b/lib/rubygems/ext/builder.rb
@@ -19,13 +19,14 @@ class Gem::Ext::Builder
     $1.downcase
   end
 
-  def self.make(dest_path, results, make_dir = Dir.pwd, sitedir = nil, targets = ["clean", "", "install"])
+  def self.make(dest_path, results, make_dir = Dir.pwd, sitedir = nil, targets = ["clean", "", "install"],
+    target_rbconfig: Gem.target_rbconfig)
     unless File.exist? File.join(make_dir, "Makefile")
       raise Gem::InstallError, "Makefile not found"
     end
 
     # try to find make program from Ruby configure arguments first
-    RbConfig::CONFIG["configure_args"] =~ /with-make-prog\=(\w+)/
+    target_rbconfig["configure_args"] =~ /with-make-prog\=(\w+)/
     make_program_name = ENV["MAKE"] || ENV["make"] || $1
     make_program_name ||= RUBY_PLATFORM.include?("mswin") ? "nmake" : "make"
     make_program = Shellwords.split(make_program_name)
@@ -131,10 +132,11 @@ class Gem::Ext::Builder
   # have build arguments, saved, set +build_args+ which is an ARGV-style
   # array.
 
-  def initialize(spec, build_args = spec.build_args)
+  def initialize(spec, build_args = spec.build_args, target_rbconfig = Gem.target_rbconfig)
     @spec       = spec
     @build_args = build_args
     @gem_dir    = spec.full_gem_path
+    @target_rbconfig = target_rbconfig
 
     @ran_rake = false
   end
@@ -191,7 +193,7 @@ EOF
       FileUtils.mkdir_p dest_path
 
       results = builder.build(extension, dest_path,
-                              results, @build_args, lib_dir, extension_dir)
+                              results, @build_args, lib_dir, extension_dir, @target_rbconfig)
 
       verbose { results.join("\n") }
 
diff --git a/lib/rubygems/ext/cargo_builder.rb b/lib/rubygems/ext/cargo_builder.rb
index 09ad1407c2..81b28c3c77 100644
--- a/lib/rubygems/ext/cargo_builder.rb
+++ b/lib/rubygems/ext/cargo_builder.rb
@@ -16,10 +16,15 @@ class Gem::Ext::CargoBuilder < Gem::Ext::Builder
     @profile = :release
   end
 
-  def build(extension, dest_path, results, args = [], lib_dir = nil, cargo_dir = Dir.pwd)
+  def build(extension, dest_path, results, args = [], lib_dir = nil, cargo_dir = Dir.pwd,
+    target_rbconfig=Gem.target_rbconfig)
     require "tempfile"
     require "fileutils"
 
+    if target_rbconfig.path
+      warn "--target-rbconfig is not yet supported for Rust extensions. Ignoring"
+    end
+
     # Where's the Cargo.toml of the crate we're building
     cargo_toml = File.join(cargo_dir, "Cargo.toml")
     # What's the crate's name
diff --git a/lib/rubygems/ext/cmake_builder.rb b/lib/rubygems/ext/cmake_builder.rb
index b162664784..34564f668d 100644
--- a/lib/rubygems/ext/cmake_builder.rb
+++ b/lib/rubygems/ext/cmake_builder.rb
@@ -1,7 +1,12 @@
 # frozen_string_literal: true
 
 class Gem::Ext::CmakeBuilder < Gem::Ext::Builder
-  def self.build(extension, dest_path, results, args=[], lib_dir=nil, cmake_dir=Dir.pwd)
+  def self.build(extension, dest_path, results, args=[], lib_dir=nil, cmake_dir=Dir.pwd,
+    target_rbconfig=Gem.target_rbconfig)
+    if target_rbconfig.path
+      warn "--target-rbconfig is not yet supported for CMake extensions. Ignoring"
+    end
+
     unless File.exist?(File.join(cmake_dir, "Makefile"))
       require_relative "../command"
       cmd = ["cmake", ".", "-DCMAKE_INSTALL_PREFIX=#{dest_path}", *Gem::Command.build_args]
@@ -9,7 +14,7 @@ class Gem::Ext::CmakeBuilder < Gem::Ext::Builder
       run cmd, results, class_name, cmake_dir
     end
 
-    make dest_path, results, cmake_dir
+    make dest_path, results, cmake_dir, target_rbconfig: target_rbconfig
 
     results
   end
diff --git a/lib/rubygems/ext/configure_builder.rb b/lib/rubygems/ext/configure_builder.rb
index 6b8590ba2e..d91b1ec5e8 100644
--- a/lib/rubygems/ext/configure_builder.rb
+++ b/lib/rubygems/ext/configure_builder.rb
@@ -7,14 +7,19 @@
 #++
 
 class Gem::Ext::ConfigureBuilder < Gem::Ext::Builder
-  def self.build(extension, dest_path, results, args=[], lib_dir=nil, configure_dir=Dir.pwd)
+  def self.build(extension, dest_path, results, args=[], lib_dir=nil, configure_dir=Dir.pwd,
+    target_rbconfig=Gem.target_rbconfig)
+    if target_rbconfig.path
+      warn "--target-rbconfig is not yet supported for configure-based extensions. Ignoring"
+    end
+
     unless File.exist?(File.join(configure_dir, "Makefile"))
       cmd = ["sh", "./configure", "--prefix=#{dest_path}", *args]
 
       run cmd, results, class_name, configure_dir
     end
 
-    make dest_path, results, configure_dir
+    make dest_path, results, configure_dir, target_rbconfig: target_rbconfig
 
     results
   end
diff --git a/lib/rubygems/ext/ext_conf_builder.rb b/lib/rubygems/ext/ext_conf_builder.rb
index fb68a7a8cc..6645c56773 100644
--- a/lib/rubygems/ext/ext_conf_builder.rb
+++ b/lib/rubygems/ext/ext_conf_builder.rb
@@ -7,7 +7,8 @@
 #++
 
 class Gem::Ext::ExtConfBuilder < Gem::Ext::Builder
-  def self.build(extension, dest_path, results, args=[], lib_dir=nil, extension_dir=Dir.pwd)
+  def self.build(extension, dest_path, results, args=[], lib_dir=nil, extension_dir=Dir.pwd,
+    target_rbconfig=Gem.target_rbconfig)
     require "fileutils"
     require "tempfile"
 
@@ -23,6 +24,7 @@ class Gem::Ext::ExtConfBuilder < Gem::Ext::Builder
 
     begin
       cmd = ruby << File.basename(extension)
+      cmd << "--target-rbconfig=#{target_rbconfig.path}" if target_rbconfig.path
       cmd.push(*args)
 
       run(cmd, results, class_name, extension_dir) do |s, r|
@@ -39,7 +41,7 @@ class Gem::Ext::ExtConfBuilder < Gem::Ext::Builder
 
       ENV["DESTDIR"] = nil
 
-      make dest_path, results, extension_dir, tmp_dest_relative
+      make dest_path, results, extension_dir, tmp_dest_relative, target_rbconfig: target_rbconfig
 
       full_tmp_dest = File.join(extension_dir, tmp_dest_relative)
 
@@ -55,7 +57,7 @@ class Gem::Ext::ExtConfBuilder < Gem::Ext::Builder
         destent.exist? || FileUtils.mv(ent.path, destent.path)
       end
 
-      make dest_path, results, extension_dir, tmp_dest_relative, ["clean"]
+      make dest_path, results, extension_dir, tmp_dest_relative, ["clean"], target_rbconfig: target_rbconfig
     ensure
       ENV["DESTDIR"] = destdir
     end
diff --git a/lib/rubygems/ext/rake_builder.rb b/lib/rubygems/ext/rake_builder.rb
index 0171807b39..42393a4a06 100644
--- a/lib/rubygems/ext/rake_builder.rb
+++ b/lib/rubygems/ext/rake_builder.rb
@@ -9,7 +9,12 @@ require_relative "../shellwords"
 #++
 
 class Gem::Ext::RakeBuilder < Gem::Ext::Builder
-  def self.build(extension, dest_path, results, args=[], lib_dir=nil, extension_dir=Dir.pwd)
+  def self.build(extension, dest_path, results, args=[], lib_dir=nil, extension_dir=Dir.pwd,
+    target_rbconfig=Gem.target_rbconfig)
+    if target_rbconfig.path
+      warn "--target-rbconfig is not yet supported for Rake extensions. Ignoring"
+    end
+
     if /mkrf_conf/i.match?(File.basename(extension))
       run([Gem.ruby, File.basename(extension), *args], results, class_name, extension_dir)
     end
diff --git a/lib/rubygems/install_update_options.rb b/lib/rubygems/install_update_options.rb
index aad207a718..0d0f0dc211 100644
--- a/lib/rubygems/install_update_options.rb
+++ b/lib/rubygems/install_update_options.rb
@@ -179,6 +179,11 @@ module Gem::InstallUpdateOptions
                "Suggest alternates when gems are not found") do |v,_o|
       options[:suggest_alternate] = v
     end
+
+    add_option(:"Install/Update", "--target-rbconfig [FILE]",
+                "rbconfig.rb for the deployment target platform") do |v, _o|
+      Gem.set_target_rbconfig(v)
+    end
   end
 
   ##
diff --git a/lib/rubygems/installer.rb b/lib/rubygems/installer.rb
index 844f292ba2..dd346cda4e 100644
--- a/lib/rubygems/installer.rb
+++ b/lib/rubygems/installer.rb
@@ -847,7 +847,7 @@ TEXT
   # configure scripts and rakefiles or mkrf_conf files.
 
   def build_extensions
-    builder = Gem::Ext::Builder.new spec, build_args
+    builder = Gem::Ext::Builder.new spec, build_args, Gem.target_rbconfig
 
     builder.build_extensions
   end
@@ -993,7 +993,7 @@ TEXT
   end
 
   def rb_config
-    RbConfig::CONFIG
+    Gem.target_rbconfig
   end
 
   def ruby_install_name
diff --git a/lib/rubygems/platform.rb b/lib/rubygems/platform.rb
index d54ad12880..9ef61ba218 100644
--- a/lib/rubygems/platform.rb
+++ b/lib/rubygems/platform.rb
@@ -12,9 +12,10 @@ class Gem::Platform
 
   attr_accessor :cpu, :os, :version
 
-  def self.local
-    @local ||= begin
-      arch = RbConfig::CONFIG["arch"]
+  def self.local(refresh: false)
+    return @local if @local && !refresh
+    @local = begin
+      arch = Gem.target_rbconfig["arch"]
       arch = "#{arch}_60" if /mswin(?:32|64)$/.match?(arch)
       new(arch)
     end
diff --git a/lib/rubygems/target_rbconfig.rb b/lib/rubygems/target_rbconfig.rb
new file mode 100644
index 0000000000..21d90ee9db
--- /dev/null
+++ b/lib/rubygems/target_rbconfig.rb
@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+require "rbconfig"
+
+##
+# A TargetConfig is a wrapper around an RbConfig object that provides a
+# consistent interface for querying configuration for *deployment target
+# platform*, where the gem being installed is intended to run on.
+#
+# The TargetConfig is typically created from the RbConfig of the running Ruby
+# process, but can also be created from an RbConfig file on disk for cross-
+# compiling gems.
+
+class Gem::TargetRbConfig
+  attr_reader :path
+
+  def initialize(rbconfig, path)
+    @rbconfig = rbconfig
+    @path = path
+  end
+
+  ##
+  # Creates a TargetRbConfig for the platform that RubyGems is running on.
+
+  def self.for_running_ruby
+    new(::RbConfig, nil)
+  end
+
+  ##
+  # Creates a TargetRbConfig from the RbConfig file at the given path.
+  # Typically used for cross-compiling gems.
+
+  def self.from_path(rbconfig_path)
+    namespace = Module.new do |m|
+      # Load the rbconfig.rb file within a new anonymous module to avoid
+      # conflicts with the rbconfig for the running platform.
+      Kernel.load rbconfig_path, m
+    end
+    rbconfig = namespace.const_get(:RbConfig)
+
+    new(rbconfig, rbconfig_path)
+  end
+
+  ##
+  # Queries the configuration for the given key.
+
+  def [](key)
+    @rbconfig::CONFIG[key]
+  end
+end
diff --git a/test/rubygems/test_gem_ext_builder.rb b/test/rubygems/test_gem_ext_builder.rb
index d5812da2aa..9907ec5f21 100644
--- a/test/rubygems/test_gem_ext_builder.rb
+++ b/test/rubygems/test_gem_ext_builder.rb
@@ -310,6 +310,65 @@ install:
     assert_path_exist @spec.extension_dir
   end
 
+  def test_build_extensions_with_target_rbconfig
+    fake_rbconfig = File.join @tempdir, "fake_rbconfig.rb"
+    File.open fake_rbconfig, "w" do |f|
+      f.write <<~RUBY
+      module RbConfig
+        CONFIG = {}
+        MAKEFILE_CONFIG = {}
+
+        def self.fire_update!(key, value); end
+        def self.expand(val, config = CONFIG); val; end
+      end
+      RUBY
+      RbConfig::CONFIG.each do |k, v|
+        f.puts %(RbConfig::CONFIG["#{k}"] = "#{v}")
+      end
+      RbConfig::MAKEFILE_CONFIG.each do |k, v|
+        f.puts %(RbConfig::MAKEFILE_CONFIG["#{k}"] = "#{v}")
+      end
+      f.puts "RbConfig::CONFIG['host_os'] = 'fake_os'"
+      f.puts "RbConfig::CONFIG['arch'] = 'fake_arch'"
+    end
+
+    system(Gem.ruby, "-rmkmf", "-e", "exit MakeMakefile::RbConfig::CONFIG['host_os'] == 'fake_os'",
+           "--", "--target-rbconfig=#{fake_rbconfig}")
+    pend "This version of mkmf does not support --target-rbconfig" unless $?.success?
+
+    @spec.extensions << "extconf.rb"
+    @builder = Gem::Ext::Builder.new @spec, "", Gem::TargetRbConfig.from_path(fake_rbconfig)
+
+    FileUtils.mkdir_p @spec.gem_dir
+
+    File.open File.join(@spec.gem_dir, "extconf.rb"), "w" do |f|
+      f.write <<-'RUBY'
+        require 'mkmf'
+
+        extconf_args = File.join __dir__, 'rbconfig_dump'
+        File.open extconf_args, 'w' do |f|
+          ["host_os", "arch"].each do |k|
+            f.puts "#{k}=#{MakeMakefile::RbConfig::CONFIG[k]}"
+          end
+        end
+
+        create_makefile 'a'
+      RUBY
+    end
+
+    use_ui @ui do
+      @builder.build_extensions
+    end
+
+    path = File.join @spec.gem_dir, "rbconfig_dump"
+
+    assert_equal <<~DUMP, File.read(path)
+    host_os=fake_os
+    arch=fake_arch
+    DUMP
+    assert_path_exist @spec.extension_dir
+  end
+
   def test_initialize
     build_info_dir = File.join @gemhome, "build_info"